A novel auxiliary difference equation (ADE) finite-difference time-domain (FDTD) method with Crank-Nicolson (CN) scheme is proposed for electromagnetic simulation of dispersive materials in this letter. The proposed method introduces an ADE technique that establishes the relationship between the electric displacement vector and electric field intensity in dispersive media with a difference equation. Hence, the CN scheme is only applied to one of the Maxwell curl equations, resulting in reduced memory usage and computing time. To validate the advantages of the proposed approach, an example with plane wave propagation in a dispersive media is presented. Compared with the conventional ADE-CN-FDTD method, the proposed method consumes much less computing time while preserves excellent simulation accuracy.

A feasible solution for mitigating radio blackout has been proposed using the magnetic field component of low-frequency (LF) electromagnetic waves (referred to as the LF magnetic field) to transmit critical low-rate data because of the low attenuation of the LF magnetic field in the electrically small plasma. Previous works have modeled the hypersonic plasma flow field as spherical or cylindrical steady-state plasma, ignoring the time-varying properties of the plasma. This article presents the dynamic propagation of the LF magnetic field in a hypersonic plasma flow field generated by a shock tube, introducing a colored-Gaussian time-varying electron density model. Analytical expressions for the propagation characteristics of the LF magnetic field in cylindrical time-varying plasma are derived using an equivalent circuit. Meanwhile, a frequency-division-multiplex experimental method for a shock tube is presented to quantitatively measure the propagation characteristics of a multifrequency LF magnetic field. The experimental results are in good agreement with the theoretical results. As the fluctuation intensity of the electron density increases, the fluctuation intensities of the time-varying envelope and phase shift increase, and the power spectral density broadens. As the frequency of the LF magnetic field increases, the effects of the time-varying plasma on the propagation characteristics strengthen. These results are important to the design of LF communication systems and modulation algorithms in the scenario of shock-tube-generated plasma.

The plasma sheath and wake flow of the hypersonic vehicle can affect the electromagnetic scattering characteristics of the reentry targets when they pass through the earth atmosphere at high speed. In order to study the similarity between the wake and the characteristic of the model launched at high velocity, the simulation experiments on the electromagnetic scattering characteristics of the spherical models made of Al2O3 and their wakes are carried out under the same binary scaling parameters in the ballistic range. The models are launched by the two-stage light-gas gun. The diameters of the models are 8 mm, 10 mm, 12 mm and 15 mm, respectively, while the pressures of the target chamber are 6.3 kPa, 5.0 kPa, 4.2 kPa and 3.3 kPa, respectively. The shock standoff distance is obtained by the shadow graph system. The electron density distribution of the wake is measured by the electron density measurement system. The RCS distribution of the wake and the model are acquired by X band monostatic radars, whose visual angle is 40. The results show that the shock standoff distance gradually increases with the increasing of the model dimension under the conditions of the same velocity and binary scaling parameters. The wake electron densities of different models are similar in their variation trends and orders of magnitude. The wake flow field of the different models with high velocity are the same as the results predicted by the double scale laws. The RCS distributions and total RCS of the wake of the models are different from each other. The electromagnetic scattering properties of the wake flow field of the various models do not conform with the predicted results obtained from the double scale law. The electromagnetic scattering energy is distributed over the regions of the models made up of aluminium oxide and the wake zones. There appears to be one center of the electromagnetic scattering energy in the area of the model coated with flow field, while several centers emerge in the region of the wake. The measuring signals of the RCS of the models show a random distribution, because the amplitude variation of the RCS and the frequency change of the RCS are random. The total RCS of the model increases with the increase of the model dimension, but the variation range of ripple frequency decreases with the increase of the model dimension.

When a hypersonic vehicle flies at high speed in near space, the distribution of key characteristic parameters of the plasma sheath covering the vehicle shows significant changes due to the influence of high-temperature gas effect and wing turbulence interference, resulting in more complicated electromagnetic (EM) transmission characteristics of the target. To more accurately study the distribution characteristics of the plasma sheath and its EM transmission properties of hypersonic vehicles under the coupling of a chemical reaction and near-wall turbulence. The computational fluid dynamics (CFD) method was used to solve the multicomponent 3-D Navier–Stokes equations with chemical reaction source terms, and the Spalart–Allmaras (S–A) turbulence model was introduced to simulate near-wall flow, numerical simulations were performed for different altitudes and velocities. The focus was on researching the distribution characteristics and evolution laws of electron density, particle mass fractions, and EM transmission in the flow field. The results show that considering the near-wall disturbance makes the peak electron density far away from the wall; the air undergoes a violent dissociation-complexity reaction after the excitation wave, the free electrons mainly originate from the contributions of oxygen and nitrogen, and the difference mainly depends on the degree of decomposition of the nitrogen. Finally, the high electron density decreases the transmission coefficient of the EM wave, increases the reflection coefficient, and increases the degree of the EM wave attenuation, and the turbulent near-wall disturbance makes the thickness of the sheath increase, which further aggravates the EM wave attenuation. This study can deepen the understanding of the EM environment of the hypersonic vehicle, and can also provide a reference for the design of the hypersonic vehicle.

Under the circumstance of rapidly fluctuating temperatures, the measurement of electromagnetic parameter of medium becomes notably more challenging due to alterations in the morphology, state of matter, and dimensions of materials resulting from ablation over a brief period. In this article, we focus on exploring a novel approach related to the measurement of time-varying complex permittivity and thickness of material under its several temperature states, with a particular emphasis on the impact of high-temperature ablation on the parameter variation of material. Initially, by measuring three typical reflection coefficients based on built single-antenna measurement system, we propose a novel measurement approach through adoption of a special cancellation technique to mitigate adverse signals that contain inherent reflection information and unknown stray signals. Subsequently, we build an explicit constraint relationship based on the definition of a generalized reflection coefficient, through which the optimization solutions of material under test (MUT) can be obtained in a well-founded manner. Furthermore, utilizing inverse chirp Z transform (ICZT) and quantum-behaved particle swarm optimization (QPSO) as techniques for obtaining the time-domain initial solutions and frequency-domain optimal solutions, respectively, we perform interval estimation of the obtained solutions to ensure their certain confidence by adopting the bootstrap strategy. The calculation results based on simulation and application of actual test data demonstrate that our proposed measurement technique and data-processing methods are effective and reliable for the measurement of time-varying complex permittivity and thickness of materials.

高超声速飞行器在临近空间飞行时，由于飞行器与空气剧烈的相互作用，形成包含等离子体鞘套和尾迹的等离子体流场，研究其电子密度分布特性对高超声速飞行器的目标识别、测控通信等具有重要意义。地面模拟实验测量是研究等离子体包覆高超声速飞行器电磁散射特性的有效方法之一，为满足地面模拟实验瞬态等离子体流场电子密度分布的测量需求，本文提出了一种Ka波段七通道微波干涉仪测量系统研制方案。该系统采用单发七收的方式，利用单曲面透镜将波导开口天线辐射的电磁波转化为近似平面波，将七个平行且非对称排列的开口波导作为接收通道天线，缩减了接收天线的尺寸以及天线之间的距离，提高了测量的空间分辨率。基于七通道微波干涉仪测量系统在弹道靶和激波管设备开展了动态实验，测量了超高速流场电子密度二维分布，结果表明该系统具备瞬时大动态范围信号的接收能力，幅度线性动态范围优于65dB，相位动态范围180°，响应时间优于1s；所测量的超高速流场等离子体电子密度二维分布，能够较好地反映弹道靶设备与激波管设备产生的瞬态等离子体细节变化，电子密度测量动态范围为（10¹⁰~10¹³）cm^-3量级，电子密度测量误差不超过0.5个数量级，径向空间分辨率优于15mm。